Optical display and projection systems may use a variety of technologies to produce images. The micromirror device has emerged as the preferred technology of optical systems. Micromirrors are small individually addressable mirrors that reflect an image. Micromirrors produce superior image quality, are lightweight, and are compact. The terms “micromirror” and “micromirror device” are used herein to refer to any micromirror technology.
Micromirror devices include a two-dimensional plane of micromirrors, called a “micromirror array,” juxtaposed in orderly rows and columns, such that an image can be reflected and/or projected. Liquid crystal on silicon (LCOS) and digital micromirror device (DMD) are two competing micromirror technologies.
LCOS and DMD devices differ in how they activate, or turn “on,” and deactivate, or turn “off,” the individual micromirrors. LCOS systems use an electrically created field to change the polarization of a liquid crystal material. The “on” and “off” states of the LCOS device depend on whether the liquid crystal is polarized or not. In a DMD device, the angle of each micromirror is changed in response to an electrical potential, producing the “on” and “off” events.
Although micromirror devices vary considerably, they are generally composed of several layers of miniaturized components. A micromirror array may include a transparent layer of glass or plastic, a liquid crystal layer (in LCOS devices), and an antireflective coating layer. A micromirror layer, including a multitude of micromirrors, is disposed such that the mirrored surfaces are oriented toward the top of the micromirror device. Each micromirror functions independently of each other micromirror. The underlying layer, or substrate, is generally composed of silicon.
A control and support region, which may be partially or completely embedded in the substrate, includes electrical circuitry for generating an electromagnetic or electrostatic field to individually control the micromirrors. Each micromirror includes a distinct control and support region, typically beneath the micromirror, which is independently operable. The components of the control and support region may vary in size and number. The small size of the micromirrors limits the available region for disposing the control and support components and, as a result, the control and support components tend to be quite small.
Since micromirror devices are used typically in high-priced systems, such as high-end computer monitors and televisions, image quality is of particular importance. The quality of the displayed or projected image depends on both the fill factor and image resolution of the display medium. For a micromirror array, fill factor refers to the percentage of the device that is occupied by micromirrors. (A similar term, fill ratio, describes the ratio of reflective areas to non-reflective areas of the micromirror array.) Thus, fill factor and fill ratio are characteristics that directly impact the quality of images produced by the micromirror array.
A device that reflects 100% of an image has a perfect fill factor. The micromirrors in prior art micromirror arrays are separated by gaps, which prevent short-circuiting between adjacent micromirrors. Since the portion of the image striking the gaps is not reflected, the reflected image is an incomplete manifestation of the original image. Because of the gaps, the fill factor of prior art micromirror devices is less than 100%. Thus, the gaps between the micromirrors impede the ability of designers to improve the image quality of micromirror devices.
In addition to the fill factor, the image resolution of micromirror devices is expected to be high. Image resolution refers to the level of detail in an image, and is measured in dots per inch (dpi) or pixels per area. Each micromirror in the micromirror array is a pixel, or picture element. Thus, an array with more micromirrors tends to have a higher image resolution.
To improve image resolution, the size of the individual micromirrors in a micromirror array may be reduced, since more of the smaller micromirrors can be disposed in the given surface area. Theoretically, the increase in the number of micromirrors improves the image resolution. As a practical matter, the size of the gaps cannot be reduced beyond a predetermined minimum size. However, as the number of micromirrors increases, the proportion of the gap spacing between micromirrors also increases, causing the fill factor to actually drop. The effort to increase the image resolution in this manner actually lowers the fill factor.
Thus, there is a continuing need for a micromirror device that improves both the image resolution and the fill factor of prior art micromirror devices.